Anti-cancer agents have various types such as an alkylating agent, a platinum agent, an antimetabolite, an antitumor antibiotic, and an antitumor plant alkaloid. These anti-cancer agents are effective for some cancers but not effective for other cancers. Even when an anti-cancer agent is confirmed to be effective for a certain cancer, the anti-cancer agent is effective for some patients and not effective for other patients, leading to interindividual differences. Whether or not a cancer of a specific patient has response to an anti-cancer agent is designated as sensitivity to the anti-cancer agent.
Irinotecan hydrochloride (CPT-11) is an anti-cancer agent which has been developed in Japan and which has a mechanism of antitumor action based on the inhibition of topoisomerase I. In Japan, CPT-11 indicated for non-small-cell lung cancer, small cell lung cancer, cervical cancer, and ovarian cancer was approved as an effective drug in January, 1994. Further, CPT-11 indicated for gastric cancer, colorectal cancer, breast cancer, squamous cell carcinoma, and malignant lymphoma was approved in July, 1995. Currently, CPT-11 in multi-drug therapy has been recognized to be one of standard chemotherapy, in particular, as a first-line or a second-line for colorectal cancer all over the world, and CPT-11 had been established the efficacy (Non-Patent Documents 1 to 6).
Meanwhile, clinical performance (including survival rate) attained by chemotherapy for advanced or metastatic colorectal cancer has been drastically improved through a combination therapy employing a key drug such as CPT-11 or oxaliplatin, which launched in 1990s, and a fluoro-pyrimidine drug such as fluorouracil (5-FU), which had been a main drug for the colorectal cancer therapy. However, the response rate of such chemotherapy is as low as about 50%. That is, the chemotherapy is not effective for half of the patients to whom an anti-cancer agent has been administered, concomitant with risky severe adverse events. Thus, there is urgent demand for establishing a marker for predicting the sensitivity to an anti-cancer agent, which marker enables determination of interindividual therapeutic response (i.e., responder/non-responder).
Although CPT-11 itself has anti-tumor activity, CPT-11 is activated by carboxyl esterase in the body, to thereby be converted into 7-ethyl-10-hydroxycamptothecin (SN-38), which has an anti-tumor activity about 100 times to some thousand times that of CPT-11. Co-presence of CPT-11 and SN-38 is thought to provide an anti-tumor effect. In hepatocytes, SN-38 is glucuronidated by UDP-glucuronosyltransferase (UGT), to thereby form SN-38 glucuronate conjugate (SN-38G) having no cytotoxicity. SN-38G is excreted mainly to bile and then transferred to the intestinal tract, and finally excreted to feces. A portion of SN-38G excreted to the intestinal tract is deconjugated by β-glucuronidase of enteric bacteria, to thereby form active SN-38 again. The thus-formed free SN-38 is metabolized and excreted via the steps of re-absorption by the mediation of a transporter present at the intestinal tract epithelium, enterohepatic circulation, glucuronate conjugation by UGT in intestinal epithelial cells, and the like (Non-Patent Document 7). In the course of this metabolism, SN-38 damages the intestinal mucosa, to thereby possibly induce diarrhea. Also, some studies revealed that SN-38 adversely affects bone marrow, where cell division actively occurs, to thereby induce erythrocytopenia, leukocytopenia, and thrombocytopenia.
One cause for adverse events such as severe diarrhea and neutropenia was confirmed to be a change in exposure amount of SN-38 in the body caused by genetic polymorphism of UGT1A1. However, regarding therapeutic effects, there has not been reported that the therapeutic effect can be predicted by pharmacokinetics, due to the complex disposition, which include conversion of CPT-11 (pro-drug) to SN-38 (active metabolite) and its detoxication; re-generation of SN-38 in the course of enterohepatic circulation; and metabolism of CPT-11 and formation of SN-38 from the metabolite thereof, and due to antitumor effect generally determined by the tumor-related factors. Meanwhile, it has been reported that the carboxyl esterase mRNA expression amount in peripheral mononuclear cells correlates with the AUC ratio of SN-38 to SN-38G but does not correlate with the tumor reduction effect (Non-Patent Document 8).
There have also been reported the following tumor-related factors relating to the sensitivity or resistance to CPT-11: mutation of topoisomerase I, which is a target of SN-38, and expression amount thereof; activity of carboxyl esterase, the enzyme being involved in transformation of CPT-11 to SN-38 (Non-Patent Document 9); and transporters (multidrug resistance protein (MRP)-1, MRP-2, and breast cancer resistant protein (BCRP)/ABCG2), which affect the intracellular accumulation of CPT-11 and SN-38. Studies have also been conducted on correlations of cell proliferation antigen Ki-67, tumor suppressor gene p53, etc. with response to CPT-11 therapy. Quite recently, in vitro, studies have been conducted to predict sensitivity to an anticancer agent systematically through combination of anti-cancer agent sensitivity data with microarray analysis data, and for camptothecin derivatives, topotecan has been studied (Non-Patent Document 10). Also, a clinical study have revealed that the plasma TIMP-1 level, TIMP-1 being a tissue inhibitor of metalloproteinase-1 having anti-apoptosis action, is significantly correlated with the clinical prognosis of a metastatic colorectal cancer patient having undergone CPT-11+5-FU combination therapy (Non-Patent Document 11).
In FOLFIRI regimen, which is a key regimen for colorectal cancer therapy, CPT-11 and 5-FU are administered in combination. 5-FU is a fluoro-pyrimidine anti-cancer agent which was developed in 1957. Even now, 5-FU is a basic chemotherapy drug for gastrointestinal cancer. When incorporated into cancer cells, 5-FU exerts cytotoxic effect through a principle action mechanism of DNA synthesis inhibition induced by inhibition of thymidylate synthase (TS) by an active metabolite, fluorodeoxyuridine-5′-monophosphate (FdUMP), and another mechanism of RNA function inhibition by an active metabolite, 5-fluorouridine triphosphate (FUTP).
Hitherto, many studies have been conducted to predict therapeutic response to fluoro-pyrimidine anti-cancer agents. In particular, many studies have been focused on dihydropyrimidine dehydrogenase (DPD), which is a 5-FU degrading enzyme, and thymidylate synthase (TS), which is a target enzyme of an active metabolite. A tumor in which DPD, a rate-limiting enzyme in the catabolism of 5-FU, is highly expressed is reported to have resistance to 5-FU (Non-Patent Document 12), but a limited number of studies have been conducted with clinical specimens. The TS expression level is reported to be a possible factor that determines the therapeutic effect by a fluoro-pyrimidine anti-cancer agent, even when the expression level is determined through any assay method such as the enzymatic activity method, protein level assay, or RNA level assay (Non-Patent Documents 13 and 14).
However, the above-obtained results are not completely the same, and there has been known no definite biomarker which can predict the therapeutic response to 5-FU in an early treatment stage.
As described above, many studies have been conducted on sensitivity (to 5-FU, CPT-11, and 5-FU/CPT-11) predicting bio-markers due to their necessity. However, a study has revealed that neither topoisomerase I (target) nor TS (possible 5-FU-sensitivity predictive factor) has clear correlation with therapeutic response in 5-FU/CPT-11 combination therapy (Non-Patent Document 15). Therefore, no definite bio-marker capable of predicting therapeutic response has been established.
Furthermore, since the therapy schedule of cancer chemotherapy generally requires a long period of time, continuous monitoring of sensitivity of a target patient to a target anti-cancer agent during the therapy can determine whether or not the therapy should be continued. Thus, such monitoring is thought to be meritorious from the viewpoints of reduction or mitigation of the burden on patients and adverse events, leading to reduction in medical cost. Therefore, there is keen demand for establishment of a biomarker that can predict the effect of 5-FU, CPT-11, or a combination of 5-FU/CPT-11 or that can determine therapeutic response in an early stage, for the purpose of predicting therapeutic response of individual patients and establishing diagnosis in an early stage to select an appropriate drug and treatment regimen; i.e., for realizing “personalized therapy.”